The search for new thermostable thermosetting polymers by addition reactions which do not produce volatile compounds during their progress are of high interest for manufacturing dense, homogeneous materials of low porosity. For this purpose, polyaddition of acetylenic groups have already been used for cross-linking easily melted and soluble monomers or oligomers. As a matter of fact, these reactions take place by mere heating of the reactants with formation of dense and cross-linked systems.
Such addition reactions have been described with aromatic, aliphatic or arylaliphatic compounds containing amide, ester, ether, sulfone, ketone, imide, phenylene and quinoxaline chainings. These reactions have been disclosed for example in reviews by P. M. Hergenrother, J. Macromol, Sci. Rev. Macromol. Chem., 1980, C19, 1, by C. Y. Lee, I. J. Goldfarb, F. E. Arnold, T. E. Helminiak, A.C.S. Polymer Preprints, 1982, 24(2), 904 and 28th Natl. SAMPE Symposium, 1983, 699, by A. O. Hanky, Natl. SAMPE Symposium, 1983, 711 and by B. A. Reinhardt, G. A. Loughran, F. E. Arnold, Polym. Sci. Technol., 1984, 25, 40. Other examples of these reactions are for example found in U.S. Pat. Nos. 4,022,746, 4,098,767, 4,100,138, 4,131,625, 4,283,551 and Belgian patent No. 898,889.
One of the major difficulties frequently encountered with thermosetting resins involving oligomers with acetylenic end groups, is their melting or softening temperature which is mostly higher than the temperature at which polymerization of acetylenic groups takes place. As a matter of fact, it has been observed that the polymerization of these groups begins at relatively low temperature, generally lower than 200.degree. C. Techniques of thermal analysis, differential calorimetry analysis and thermomechanical analysis show that the polymerization thermal threshold is often in the range from 120.degree. C. to 180.degree. C. In these conditions, it is preferable, for controlling the polymerization process, to use compositions having a melting or softening temperature substantially lower than the temperature prevailing at the beginning of the polymerization.
Aromatic polyethers, particularly those whose aromatic rings are interconnected through oxygen atoms in meta position, are known as having rather low melting temperatures since some of them are used as thermostable fluids. But the formation of ether bonds between aromatic carbon rings involved condensation reactions occurring at high temperature which requires the presence of catalytic systems essentially comprising copper.
Halogen atoms in ortho or para position from a heterocyclic nitrogen atom are known as being much more reactive than those fixed on the carbon atoms of a carbon ring. Among the nitrogenous heterocyclic compounds which have a high resistance to heat and oxidation, pyridine has the advantage of being industrially available. Accordingly, its derivatives with halogen on positions 2 and 4 or 2 and 6 constitute convenient raw materials for synthesizing oligomers of aromatic-heterocyclic polyethers.